1. Field of the Invention
The present invention relates to data communication, and more particularly, to a data communication apparatus and method based on orthogonal frequency division multiple access (OFDMA).
2. Description of the Related Art
In conventional data communication based on OFDMA, information is transmitted from a base station to a user part in units of data symbol blocks as follows. First, N-point inverse discrete Fourier transform (IDFT) or N-point inverse fast Fourier transform (IFFT) is performed on information to be transmitted (N denotes a total number of symbols to be allocated to users). A symbol block generated by adding a cyclic prefix to the front of a sequence of main symbols having transformed information is transmitted. As a result, in conventional data communication based on OFDMA, an entire frequency band is divided into N small frequency bands (or sub-carriers), and a single data symbol is transmitted through each sub-carrier.
In conventional data communication based on OFDMA, a base station deduces user symbols from a signal received from a user part as follows. First, the start point of each symbol block is found, and a cyclic prefix is removed. Then, N-point DFT or N-point FFT is performed on N samples to convert the N samples into a signal in a frequency domain. Next, signal distortion occurring in each frequency due to a channel is compensated for, and data is detected. Here, a cyclic prefix is added to prevent inter-block interference from occurring due to a channel and needs to be longer than an impulse response of the channel. Here, the value of N needs to be as large as possible in order to increase the efficiency of conventional data communication based on OFDMA. However, with an increase in the value of N, the size of hardware for FFT/IFFT also increases, so it is difficult to realize conventional data communication based OFDMA.
In normal data communication based on OFDMA, when a plurality of users communicate simultaneously, as many frequencies or sub-carriers as needed are allocated to each user to establish multiple access. Accordingly, each user part transmits data symbols using only some frequencies, and the remaining frequencies are not used for data transmission. Even in this case where data is transmitted at only some frequencies, a user part in conventional data communication based on OFDMA must perform N-point IFFT and N-point FFT, which results in unnecessary waste of hardware.
In a multimedia wireless communication environment in which various speeds and characteristics from a low-speed voice service to a high-speed data service exist together, a user part, that is, a user terminal, including a user transmitting unit and a user receiving unit provides only limited services according to its price or purpose. For example, a low-priced terminal can provide only a low-speed data service such as a voice service, and a high-priced terminal can provide a high-speed data service and a video service. Accordingly, although N should be set to be large in order to increase communication efficiency in the multimedia wireless communication environment, N can be set to be small for a low-speed terminal. However, conventional data communication based on OFDMA complicates the hardware and designs of terminals.